A laser is a device which has the ability to produce coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices are referred to as laser diodes and, as used herein, the term laser diode includes laser diode arrays.
By appropriate selection of the laser diode composition, it is possible to produce a device which emits output radiation at substantially any wavelength over the range from about 630 to about 1600 nm. For example, the wavelength of the output radiation from a InGaASP based device can be varied from about 750 to about 900 nm by variation of the device composition. Similarly, the wavelength of the output radiation from an InGaAsP based device can be varied from about 1000 to about 1600 nm by variation of the device composition.
The conversion of optical radiation of one frequency to optical radiation of another frequency through interaction with a nonlinear optical material is well-known and has been extensively studied. Examples of such conversion include harmonic generation, optical mixing and parametric oscillation.
Materials having nonlinear optical properties are well-known. For example, U.S. Pat. No. 3,949,323 issued to Beirlen et al. on Apr. 6, 1976, discloses that nonlinear optical properties are possessed by materials having the formula MTiO(XO.sub.4) where M is at least one of K, Rb, Tl and NH.sub.4 ; and X is at least one of P or As, except when NH.sub.4 is present, then X is only P. This generic formula potassium titanyl phosphate, KTiOPO.sub.4, a particularly useful nonlinear material. Other known nonlinear optical materials include, but are not limited to, KH.sub.2 PO.sub.4, LiNbO.sub.3, KNbO.sub.3, .beta.-BaB.sub.2 O.sub.4, Ba.sub.2 NaNb.sub.5 O.sub.15, LiIO.sub.3, HIO.sub.3, KB.sub.5 O.sub.8. 4H.sub.2 O, potassium lithium niobate and urea. A review of the nonlinear optical properties of a number of different uniaxial crystals has been published in Sov. J. Quantum Electron., Vol. 7, No. 1, Jan. 1977, pp. 1-13. Nonlinear optical materials have also been reviewed by S. Singh in the CRC Handbook of Laser Science and Technology, Vol. III, M. J. Weber, Ed., CRC Press, Inc., Boca Raton, Fla., 1986, pp. 3-228.
Second-harmonic generation or "frequency doubling" is perhaps the most common and important example of nonlinear optics wherein part of the energy of an optical wave of angular frequency .omega. propagating through a nonlinear optical material is converted to energy of a wave of angular frequency 2 .omega.. Second-harmonic generation has been reviewed by A. Yariv in Quantum Electronics, Second Ed., John Wiley & Sons, New York, 1975 at pages 407-434 and by W. Koechner in Solid State Laser Engineering, Springer Verlag, New York, 1976 at pages 491-524.
As used in this application, the term "optical mixing" refers to the interaction within a nonlinear optical material of two beams of light having frequencies .omega..sub.1 and .omega..sub.2 to produce optical radiation of a different frequency. For example, where .omega..sub.1 is greater than .omega..sub.2, this interaction can produce optical radiation at the sum-frequency, .omega..sub.3 =.omega..sub.1 +.omega..sub.2, and at the difference-frequency, .omega..sub.4 =.omega..sub.1 -.omega..sub.2. These two processes are referred to as sum-frequency generation and difference-frequency generation, respectively. Up-conversion refers to the special case of sum-frequency generation where radiation of one frequency, for example ml, is much more intense than that at .omega..sub.2 and, accordingly, does not undergo any appreciable change in amplitude as optical mixing occurs to give optical radiation of wavelength .omega..sub.3. Optical mixing also includes higher order processes such as .omega..sub.5 =.omega..sub.1 +2.omega..sub.2 and .omega..sub.6 =2.omega..sub.1 -2.omega..sub.2. For the purposes of this application, the optical radiation produced by optical mixing is generically referred to as "optical mixing radiation."
The frequency conversion of optical radiation by a nonlinear optical material can be carried out either within or outside of an optical cavity. If the process is carried out within an optical cavity, that cavity can be either: (a) a component of one of the sources of radiation for the process, or (b) separate from any cavity utilized as a component of any source of radiation for the process. For convenience, the use of such a source cavity will hereinafter be referred to as an intracavity process and the use of a separate cavity will be referred to as an external cavity process. For the purposes of this application, an optical cavity or resonator refers to a volume, which is bounded at least in part by highly reflecting surfaces, wherein light of certain discrete frequencies can set up standing wave modes of low loss.
The up-conversion of infrared radiation to the visible and ultraviolet range has been extensively studied. Such studies have been primarily motivated by an interest in using this technique to permit the detection and analysis of infrared radiation by the conventional and efficient methods that are available for light of higher frequency. Since the up-converted radiation carries essentially all of the information of the input infrared radiation, potential applications include infrared signal detection, infrared spectral analysis and infrared holography.
Up-conversion of infrared radiation has been reviewed by E. S. Voronin et al., Sov. Phys. Usp., Vol. 22, No. 1, pp. 26-45 (Jan. 1979) and J. Warner, "Difference Frequency Generation and Up-Conversion" in Quantum Electronics, Vol. I, Nonlinear Optics, Part B, H. Rabin and C. L. Tang, Ed., Academic Press, New York, pp. 703-737 (1975). A theoretical discussion of infrared detection by sum-frequency generation has also been published by D. A. Kleinman et al., J. Appl. Phys., Vol. 40, No. 2, pp. 546-566 (Feb. 1969).
At page 34 of their previously-cited review article, E. S. Veronin et al. describe the up-conversion of infrared radiation from a CO.sub.2 laser within the cavity of a YAG:Nd.sup.3+ laser using proustite as the nonlinear optical material. In addition, E. Liu et al., Applied Optics, Vol. 21, No. 19, pp. 3415-3416 (1 Oct. 1982) have reported the generation of radiation at wavelengths in the range from 252 nm to 268 nm by intracavity sum-frequency generation in a 90.degree. phase-matched temperature-tuned ammonium dihydrogen phosphate crystal, of selected output lines from an argon ion laser and the traveling wave in a rhodamine 110 ring dye laser. Further, U.S. Pat. No. 3,646,358, issued to Firester on Feb. 29, 1972, discloses the up-conversion of signal radiation from an external source within the cavity of a laser wherein the polarization of the signal beam is orthogonal to that of the pump beam which is generated within the laser cavity.
At pages 559-564 of their above-cited review article, D. A. Kleinman et al. have discussed the theoretical aspects of sum-frequency generation in an external cavity. In addition, V. L. Aleinikov et al., Sov. J. Quantum Electron., Vol. 13, No. 8, pp. 1059-1061 (Aug. 1983), have analyzed the theoretical aspects of parametric up-conversion in an external cavity. Further, H. Hemmati et al., Optics Letters, Vol. 8, No. 2, pp. 73-75 (Feb. 1983), have reported the generation of radiation at a wavelength of 194 nm by sum-frequency generation in an external cavity using as input radiation: (a) the 257 nm second harmonic of the output of a continuous wave (cw) 515 nm argon-ion laser, and (b) the output of a tunable cw dye laser in the 792 nm region.
Difference-frequency generation has been reviewed in the above-cited review article in Quantum Electronics, 5 Vol. I, at pp. 735-736 and by R. L. Aggarwal et al. in Nonlinear Infrared Generation, Y.-R. Shen, Ed., Springer verlag, Berlin, pp. 19-38 (1977).
Dahmani et al. have reported in Optics Letters, Vol. 12, No. 11, pp. 876-878 (Nov. 1987) that a separate Fabry-Perot cavity can be used to provide optical feedback to a single mode, 850 nm GaAlAs laser diode that forces the laser diode to lock its frequency to that of the cavity resonance. As a consequence, the frequency of the diode laser is stabilized and the linewidth of the laser is reduced by a factor of 1000 from 20 MHz to approximately 20 kHz.
There is a current need for efficient, compact and reliable lasers which operate in the infrared, visible and ultraviolet portion of the spectrum and are capable of modulation rates over the range from 0 Hz to in excess of 1 GHz over a wide range of intensities. Such devices would be useful for applications which include optical storage of data, reprographics, spectroscopy and communications. For example, the storage of data on optical disks requires a source of coherent radiation which can be modulated at a rate between about 5 and about 20 MHz, and such radiation is desirably in the visible or ultraviolet portion of the spectrum in order to maximize data storage within a given area. In addition, compact coherent sources of red, green and blue light would be highly attractive for television applications requiring a high brightness source. The use of three such lasers in place of the red, green and blue electron guns of a conventional television picture tube would result in a high brightness television projector that would be useful in simulation systems and large screen television systems. Laser diodes possess all of the above-described capabilities except for one--their output is in a limited part of the electromagnetic spectrum at wavelengths in the range from about 630 nm to about 1600 nm.